In recent years, hybrid vehicles (HEV), electric vehicles (EV) and fuel cell vehicles have been manufactured and sold in view of environmental effects and fuel consumption, and further development of these vehicles is being carried out. In such electric-powered vehicles, the use of chargeable-dischargeable power supply systems is essential. As for such power supply systems, secondary batteries such as lithium ion batteries and nickel hydride batteries, and electric double-layer capacitors are generally used. Among these, lithium ion secondary batteries are particularly suitable for used in electric-powered vehicles because of high energy density and high resistance to repeated charge and discharge, and recent batteries tend to have much higher capacity.
Such lithium ion secondary batteries have a configuration in which a positive electrode and a negative electrode are connected via an electrolyte layer and these are housed in a battery case. The electrolyte layer may include a separator in which an electrolysis solution is held. The separator is required not only to hold the electrolysis solution to ensure lithium ion conductivity between the positive electrode and the negative electrode but also to serve as a partition between the positive electrode and the negative electrode.
Separators are being developed that have a shutdown function to stop a charge-discharge reaction when batteries reach high temperature. The shutdown function is to prevent movement of lithium ions between the electrodes. In particular, when a battery reaches high temperature, resin contained in the separator is dissolved and pores are then filled with the resin so that a shutdown is carried out. Therefore, the material used in the separator having such a shutdown function is generally thermoplastic resin such as polyethylene (PE) or polypropylene (PP).
Here, it is well known that such a separator containing thermoplastic resin has a problem with mechanical strength due to the flexibility of the material. In particular, the separator is thermally contracted under high temperature conditions, and the positive electrode and the negative electrode interposing the separator therebetween come into contact with each other so that an internal short circuit may be occurred. In view of such a problem, there is proposed a method of forming a heat resistant insulation layer containing insulation inorganic particles and an organic binder as main components and interposed between the separator and the respective electrodes.
Such a separator may have influence on cycle performance and output performance when applied to a battery because the adsorption moisture amount in the separator is higher than that of a separator only including a porous substrate due to the presence of the heat resistant insulation layer containing the inorganic particles and the binder. With regard to measures to control a moisture content in a separator with a heat resistant insulation layer, International Publication WO 08/029,922 discloses a method of performing hydrophobic treatment on surfaces of inorganic fine particles. Japanese Patent Unexamined Publication No. 2010-232048 also discloses a method relating thereto.